Neutron-optical componet array for the specific spectral shaping of neutron beams or pulses

ABSTRACT

A neutron-optical component array in which the beam paths of the individual moderators are combined in a concerted manner so as to create a superimposed neutron beam with an effective mean beam direction. The superimposed neutron beam has a multi spectrum composed of the single spectrums of several moderators, whereby a larger spectral width is obtained, making various applications in different neutron energy fields possible. The multi spectrum can be further improved in terms of the intensity thereof and the beam quality by adding further neutron-optical components, particularly in the form of an energy-depending switching super reflector, and by switching between moderators.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a neutron-optical component array for thespecific spectral shaping of neutron beams or pulses in a neutron guideor beam hole between a fast neutron source with several moderators ofdifferent structures arranged closely adjacent each other for generatingslow neutrons of different energy spectra as well as for their radiationin predetermined radiation directions and to at least one place ofexperiment.

2. The Prior Art

Neutron beams serve in a broad spectrum of scientific examinationsranging from pure basic science to application-related examinations inthe field of research of the structure of matter. Here, neutronsfunction quasi as sensors which penetrate into the matter. Neutronsimpinging upon atoms of structured matter are either scattered in amanner characteristic of the atoms or they are absorbed by the atoms byemitting characteristic radiation. For most applications, as forinstance in neutron scattering, it is necessary to provide slow neutronswhich are generated by deceleration of fast neutrons obtained fromnuclear reactions. Intensive neutron radiation of fast neutrons isprimarily generated in research reactors either by splitting enricheduranium in a temporally constant flow or as pulses in spallation sourcesby crushing heavy atoms.

The specific deceleration of fast neutrons is primarily carried out byso-called “moderators” which are brought into contact with the fastneutron radiation. Stated in simple terms, these are collections ofmatter of gaseous, liquid or solid appearance which, at a predeterminedtemperature, have specific characteristics. By the interaction of fatneutrons with the preferably light atoms of the moderator matter, thehigh energetic neutrons are strongly decelerated to the point wheretheir energies and wavelengths are of the requisite values forexperiments with condensed matter. A neutron gas of kinetic energydistribution is produced which at a given temperature may beapproximated by a Maxwellian velocity distribution. This is atheoretically derived function which assigns their relative abundance tothe velocities of the atoms of a gas. The effective temperature of theMaxwellian spectrum of the neutron gas is somewhat higher, however, thanthe temperature of the moderator matter. In this connection it is to bementioned that neutron reflectors such as, for instance, (heavy) water,lead, beryllium, graphite, etc. also generate slow neutrons, but with aspectrum different from the spectrum which may be approximated by theMaxwell spectrum. Nevertheless, reflectors which serve primarily toincrease the flow of neutrons also contribute to neutron-deceleration,so that, in a broader sense, they may, as neutron-optical components, begrouped with the moderators. Premoderators such as water and all otherstructures of a neutron sources capable of emitting slow neutrons mayalso be counted among the group of moderators.

Depending upon the temperature of the moderator material, slow neutronsare differentiated between “hot”, “thermal”, and “cold” neutrons, sothat the moderators may also be distinguished as “hot”, “thermal”, and“cold” moderators. In the present context, slow neutrons are those of akinetic energy in the range of 1 eV and less. The energy of hot neutronsof higher velocity and lesser wavelength is in a range above 100 meV andare particularly suitable for scatter experiments with liquids. Thermalneutrons are of a kinetic energy in the range of between 10 meV and 100meV, and the kinetic energy of cold neutrons lies in the range between0.1 meV and 10 meV. Cold neutrons of relatively low velocity and largewavelength are above all of importance for applications of neutronscattering for examining biological substances. Depending on the kind oftheir primarily generated slow neutrons, a distinction is made betweenhot, thermal and cold moderators. A survey of possible moderatorstructures in a spallation source may be derived from paper I “ParticleTransport Simulations of the Neutron Performance of Moderators of theESS Mercury Target-Moderator-Reflection System” (downloadable from theInternet at http://www.hmi.de/bereiche/SF/ess/ESS_moderators3.pdf, state18 Jan. 2002). Examples thereof are the liquid hydrogen moderator withan operating temperature in the range of 25° K for generating coldneutrons and the water moderator using the ambient temperature as itsoperating temperature for generating thermal neutrons. However, a coldmoderator also generates thermal and hot neutrons as well, and a thermalmoderator also generates cold and hot neutrons, but always at a flowlower by an order of magnitude than the moderator which serves forgenerating primarily cold, thermal or hot neutrons.

To provide the correct required neutron spectrum for differentexperiments with slow neutrons, the known neutron sources operate with acombination of different moderators. From Paper II “The SpallationNeutron Source Project” by Jose R. Alsonso; Proceedings of the 1999Particle Accelerator Conference, New York, 1999, pp. 574-578,(downloadable from the Internet athttp://accelconf.web.cem.ch/accelconf/p99/PAPERS/FRAL1.pdf—(State 18.Jan. 2002), it is known to position two water moderators tempered byroom temperature below the level with the target material to be crushedand two super-critical hydrogen moderators with an operating temperatureof 20° K above the target plane. Each moderator exclusively provides oneor more of eighteen places of experiment with the slow neutron spectrumgenerated by it (see FIG. 9 and Chapter 6 of Paper II). A similarstructure is also known from Paper III “5.3—Material Issues forSpallation Target by GeV Proton Irradiation” by W. Watanabe(downloadable from the Internet athttp://www.ndc.tokai.jaeri.go.jp/nds/proceedings/1998/watanabe_n.pdf;state 18 Jan. 2002). It describes a target-moderator-configuration forexecuting high intensity and high resolution experiments with coldneutrons, in which a coupled cold moderator with a premodulator and twothermal moderators are arranged closely adjacent the target in theregion of the highest and fastest neutron radiation (see Paper III,Chapter 4 (2) to (4) and FIG. 2). As an important point, the paperrefers to the close proximity notwithstanding, cross-talk between theindividual moderators which effects the neutron intensity, can beprevented (see Paper III, Chapter 4 (ii)). For that reason, themoderators are arranged relative to each other at such angles that theirforward and rearward radiation directions or emitted neutron beams areoriented in different spatial directions without overlapping each other.In this manner, each moderator supplies about four to eight places ofexperiment with a neutron beam of characteristic spectrum. Moreover,reflectors are arranged between the to levels for separating thespectra.

Proceeding from the known state of the art relating to the knownapplication of moderators as described, for instance, in previouslycited Paper III, it can be recognized that the provision of a neutronspectrum of slow neutrons required for a specific experiment as well asthe generation thereof causes significant problems. In particular, withregard to the very complex and expensive structures of theneutron-optical components as well as the high protective measures whichthey require, the state of the art knows of no neutron spectrum for asingle place of experiment. Each place is supplied with a neutronspectrum the maximum of which indicates the principally generated slowneutrons, from a directly associated moderator type. Changes in thespectrum of the neutron beam at a place of experiment may be realizedonly by significant structural changes in the structure of the moderatorat extended down-times of the neutron source. Experiments in energyranges broader than the one of a single slow neutron form are notpossible or they are very inefficient.

OBJECTS OF THE INVENTION

For that reason, it is an object of the invention to provide an array ofneutron-optical components for the specific shaping of the spectrum of aneutron beam of the kind referred to supra which offers significantflexibility in respect of providing one neutron beam to one place ofexperiment, so that no extensive structural changes are required in caseof change requirements. More particularly, experiments with neutronsfrom a larger energy range are to be made possible as well. Furthermore,the neutron beam provided by the invention is to be of high quality. Themeans for realizing the invention are to be simple in their structureand operation and, therefore, subject to relatively few malfunctions aswell as low costs. Present aspects of safety are to be taken intoconsideration and additional risks are to be avoided.

BRIEF SUMMARY OF THE INVENTION

In the accomplishment of this object the invention provides in aneutron-optical component array for the specific shaping of neutronbeams or pulses of the kind described hereinbefore for the radiationdirections of the moderators to overlap directly or by furtherneutron-optical components in the neutron guide or at the place ofexperiment and for the slow neutrons of different energy spectra in anoverlapping neutron beam be detected together with a multi-spectrumwhich is defined by the structure and number of moderators used.

The energy spectra of different moderators are combined into a“multi-spectrum” by the neutron-optical component array in accordancewith the invention. A neutron beam (or a neutron pulse—this alternativeis always to be included when the term “neutron beam” is used) with sucha multi-spectrum may be used in many different applications. As it has abroader energy spectrum than the individual neutron beams generated by amoderator, the overlapping neutron beam in accordance with the inventionmakes possible neutron experiments with high efficiency in a broadenergy range of the impinging neutrons, e.g. between 0.1 meV and 100meV. The composition of the multi-spectrum of the overlapping electronbeam depends upon kind and number of moderators used. For instance, acold and a thermal moderator or a cold, a thermal, and a hot moderatormay be combined in their direction of propagation. In the same manner,different designs of a type of moderator may be combined to achieve aparticularly broad multi-spectrum or a specially-formed multi-spectrumin terms of its emission. The combination of different modulators islimited only by structural restraints since in terms of apparatustechnology the combination of the radiation direction must be realizablewith a reasonable effort. In this connection, mention is to be made thatother neutron-optical components present in the neutron system as wellas parts of the neutron source itself may, of course, be included in thecomposition of the multi spectrum, with other main functions whichprovide for a decelerating effect on the neutrons, such as reflectors,neutron guides, and primary moderators, by combining the emittedradiation into the common neutron beam. This results in a single ormultiple overlapped neutron beam for many different applications. Thepoint of gravity of the invention resides in the combination of theindividual neutron beams in a common neutron beam with a correspondinglybroadened energy spectrum. Heretofore, the prior art has alwaysproceeded from an express and deliberate separation of the effectiveranges of the moderators since this seemed to be the only possibilitywithout much effort to provide suitable slow neutron beams for yieldingusable measurement results. The disadvantage of the low flexibility wasaccepted and corresponding numbers of places of experiment wereconceived.

The overlapping of the individual neutron beams from the moderators usedto a common neutron beam may take place in the neutron guide as well atthe place of experimenting. The first case results in the formation of aneutron beam which like a single electron beam is conducted in oneneutron guide to the place of experiment and to the probe. In the secondcase, the different neutron beams are focused on the probe to beexamined so that the overlapping neutron beam impinges directly on theprobe. The advantage of this overlapping irradiation at the place ofexperiment itself resides in the relatively low technical complexity forcombining the directions of radiation of the individual moderators. Inthe simplest case, the adjacent moderators are to be arrayed relative toeach other at such angles that it results in a focal point of theradiation directions in the probe or slightly in front thereof. In afurther development of the neutron-optical component array in accordancewith the invention the radiation directions may, in case they overlapdirectly, be detectable at the place of experiment by a predeterminedencoding scheme. In terms of the measurement results it may be importantto know the different radiation directions from which the differentkinds of neutrons impinge upon the probe. In a pulsed neutron sourcethis may be carried out by monitoring the neutron flight time. In caseof a it is necessary to chop the neutron beam correspondingly. Sincewithin the slow neutrons, the cold, thermal, and hot neutron differ bythe energy spectrum and, hence, by their velocity distribution,knowledge of the individual neutron flight times makes possible, on thebasis of the pulses, an association to the individual moderators and,therefore, with their radiation direction relative to the probe.

However, for the majority of applications in experiments it is importantthat all the neutron from a common spatial direction impinge upon theprobe to be examined. This common spatial direction will hereafter bedenominated “effective mean beam direction”. To achieve a common beamdirection overlapping of the individual neutron beams by furtherneutron-optical components is necessary. Different components are knownfor the specific control of the neutron beams, all of which are suitablein the array in accordance with the invention to bring about acombination of the emissions of the moderators. Among these is theneutron guide itself which in accordance with one embodiment of theinvention may on its interior surface be plated with nickel (se Germanpatent specification DE 44 23 781 A1) and which reflects neutronimpinging at predetermined especially flat angles into the interior ofthe tube. If two neutrons impinge the input section of the neutron guidefrom two different directions, for instance, they will be steered intothe desired effective mean beam direction during the course of theneutron guide by the internal reflection thereof.

Furthermore, in an overlapping of the radiation directions by furtherneutron-optical components for achieving an effective mean beamdirection of the overlapping neutron beam, a further embodiment of theinvention may provide for a further neutron-optical component structuredas an oscillating reflector which oscillated in synchronism with apulsed neutron source or with the chopped neutron beam of a continuousneutron source. The oscillating reflector causes the neutron beams fromdifferent moderators to be alternatingly inserted into the overlappingneutron beam with the effective mean beam direction. If, for instance,the reflector oscillates to and fro between a cold and a thermalmoderator at the beat rate of a neutron pulse source and if its angle isproper in respect of the impinging cold neutrons, it will initiallyreflect the cold neutron pulse into the means radiation direction.Thereafter, the angle of the reflector is changed at the beat rate ofthe pulse so that thermal neutrons will impinge and the thermal neutronpulse is coupled in. The respective other neutron pulse will bedeflected outside of the mean radiation direction. At a continuousneutron beam from a core reactor mechanical or chopper arrangementsoperating differently may be used for chopping the continuous neutronbeam into individual pulses. In such an embodiment, measurements at theprobe are to be carried at the beat rate of the neutron pulses or of theoscillator.

It has already been mentioned supra that in the energy spectra of theindividual moderators two marginal areas with neutron energies occurwhich are mainly generated by the other moderators. If in an experimentonly cold neutrons have been fed to a probe, hot and thermal neutronswill nevertheless be present in the neutron beam, yet at a significantlylower quantity. In accordance with a further embodiment of theneutron-optical component array in accordance with the invention it isparticularly advantageous to provide a further neutron-optical componentwith an energy depending switching function. In this variant of anembodiment, there is no active moving reflector switching back and forthbetween individual neutron beams, but a neutron-optical system isprovided instead which simultaneously captures all impinging neutron. Inthis connection a neutron-optical component is used which is providedwith an energy-selective switching function. Such components may bestructured and aligned so that they pass, for instance, the centralenergy range of each moderator with the greatest quantity of theneutrons to be generated and couple them into the effective meanradiation direction. By contrast, they block the marginal areas with theenergetically diverging neutrons. The multi spectrum of the overlappingneutron beam may be combined by the switching function by passing forthe individual kinds of neutrons the corresponding neutrons from themoderators which generate them. It is thus possible for cold as well asfor thermal and hot neutrons to attain a maximum neutron flow for theexperiments.

Neutron-optical components with an energy-selective switching functionmay be realized primarily by special neutron reflectors. For thatreason, a further embodiment of the invention provides for the furtherneutron-optical component with an energy-depending switching function tobe structured as a neutron reflector which continuously orintermittently passes or blocks impinging neutron by a correspondingangular alignment depending upon their energy. For further explainingthe functional cooperation of the neutron reflectors, to achieve theswitching action described above, reference may be had, for the sake ofavoiding repetition, to the particular section of this specification. Inaccordance with a further embodiment of the invention, the neutronreflectors may advantageously be structured to be self-supporting or asbeing applied on a neutron-transparent substrate as a single layer ormulti-layered reflector, with the coating being applied to one or bothsides of the substrate. The multi-layered neutron reflectors areso-called “super-reflectors” with interfering properties (see Germanpatent specification DE 198 44 300 A1). For instance, silicon andsapphire are suitable substrates. All of these neutron-opticalcomponents are of relatively simple structure and are thus inexpensivecompared to other neutron-optical components. A particularlyadvantageous and compact structure of the invention results inaccordance with another embodiment by integrating the furtherneutron-optical components with an energy-depending switching functioninto the neutron guide. As regards this embodiment, reference may behad, for the sake of avoiding repetition, to the specific portion of thedescription.

DESCRIPTION OF THE SEVERAL DRAWINGS

The novel features which are considered to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, in respect of its structure, construction andlay-out as well as manufacturing techniques, together with other objectsand advantages thereof, will be best understood from the followingdescription of preferred embodiments when read in connection with theappended drawings, in which:

FIG. 1 depicts a neutron-optical component array for generating a multispectrum; and

FIG. 2 depicts the switching function provided by the system of FIG. 1for generating a multi spectrum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts the neutron-optical component array NOA for the specificspectral shaping of neutron beams or pulses. In the selected embodimenta cold moderator CNM for neutrons is arranged closely adjacent a thermalmoderator TNM for neutrons. Both moderators CNM, TNM measure 12×12 cm incross-section and are separated by a gap of 0.5 cm. Instead of arepresentation of an angular arrangement between the two moderators CNM,TNM their radiation directions CBL, TBL are indicated as being angularrelative to each other. The cold moderator CNM emits a neutron spectrumhaving a maximum of cold neutrons CCN and a smaller proportion ofthermal neutrons CTN. On the other hand, the thermal moderator TNMgenerates a maximum of thermal neutrons TTN and a lesser proportion ofcold neutrons CTN. The thermal moderator TNM is arranged directlyopposite a neutron guide NGT which conducts the coupled-in neutrons to aplace of experiment not shown in FIG. 1. The neutron guide NGT has across-section of 6 cm×6 cm and extends from the neutron source also notshown in FIG. 1 by a distance of 32 m. For improving its reflectiveproperties it is coated with nickel on its internal surface INS. Bymultiple flat reflection of acutely impinging neutron beams CCN, TTN itconcentrates them in an effective mean radiation direction EBL to anoverlapping neutron beam SBL having a multi spectrum. By attaining theeffective mean radiation direction EBL, the neutrons impinge upon theprobe to be analyzed quasi from one direction.

The overlapping neutron beam SBL generated in the neutron guide NGT bybeam overlapping has a multi spectrum of particularly high value whichis composed of from the maximum ranges of the spectra only of the twomoderators CNM, TNM. To obtain such a purified multi spectrum which maybe used with particular advantage for experiments in a broad energyrange, further neutron-optical components NOC with an energy-dependentswitching function are integrated into the neutron guide NGT at its endfacing the two moderators CNM, TNM at a distance of 1.5 m therefrom. Inthe selected embodiment, these are a simple neutron conducting superreflector RSM and a further super reflector SSM opposite the first one.They arranged at an angle of 0.72° relative to the direction of theneutron guide NGT. So that the super reflector SSM reflects or passesimpinging neutrons as a function of their kinetic energy. If a differentangle is selected, the other dimensions of the participating componentsmust be changes correspondingly. Both super reflectors RSM, SSM have alength of 6.5 m and are of commercial quality m=3, i.e. their sectionalangle is thrice the sectional angle of natural nickel. The superreflector SSM is applied at a thickness of 0.75 mm to a neutrontransparent Si substrate. Whereas the super reflector RSM serves merelyto reflect emitting neutron beams, the opposite super reflector SSMfulfills an energy and angle depending switching function. In theselected example, the super reflector SSM is constructed and set in itsangle (for instance 0.72° in this example) such that it reflects thecold neutrons CCN of the cold moderator CNM into the neutron guide NGT,whereas the cold neutrons CTN from the thermal moderator TNM arereflected away from the area of the neutron guide NGT by the other sideof the reflector. In the opposite case, the thermal neutrons TCN of thecold moderator CNM are guided out of the neutron guide NGT along thesuper reflector SSM, whereas the thermal neutrons TTN from the thermalmoderator TNM may unimpededly pass through the super reflector SSM. Inthis manner the overlapping neutron beam SBL is composed ofpreferentially emitted neutrons from both moderators CNM, TNM. Thisensures on the one hand that at every neutron energy switching takesplace to the moderator with the higher neutron flow and, on the otherhand, that the other moderator with the possibly lesser beamquality—e.g. pulse shape in case of pulsed sources—are deflected out.

FIG. 2 depicts the switching function for generating the multi spectrumof the arrangement in accordance with the invention in exemplarilyselected embodiment of FIG. 1. The relative transmission coefficient RTCof the entire neutron-optical system is shown as a function of theneutron wavelength NWL in nm for bother moderators CNM, TNM of FIG. 1and may be defined as by comparison with the simple spectra in anidentical neutron guide which is arranged at a distance of 1.5 m eitherahead of the cold or ahead of the thermal moderator CNM, TNM. If neutronenergy greater than 20 meV (this corresponds to a neutron velocity inexcess of 2,000 m/sec or, by way of equivalence, to a neutron wavelengthbelow 0.2 nm) are needed in an experiment, thermal neutrons TTNexclusively will be available in the combined multi spectrum. At neutronenergies less than 5 meV (corresponding to a neutron velocity of lessthan 1,000 m/sec or, by way of equivalent, to a neutron wavelength ofmore than 0.4 nm) the supply of neutrons is satisfied with cold neutronsCCN almost exclusively from the cold moderator CNM. In a transitionalrange between 5 meV and 20 meV the neutrons TTN, CCN are fed in theoverlapping neutron beam SBL to the experiment from both moderators TNM,CNM as a mixture with different proportions.

1. A neutron-optical component array for the specific spectral shapingof neutron beams or pulses in a neutron guide or in a radiation holebetween a fast neutron source with a plurality of moderators ofdifferent structures arrayed closely together for generating slowneutrons of different energy spectra as well as for their radiation inpredetermined radiation directions and at least one place of experiment,characterized by the fact that the radiation directions (CBL, TBL) ofthe moderators (CNM, TNM) are overlapped directly or by furtherneutron-optical components (RSM, SSM) in the neutron guide (NGT) or atthe place of experiment and that the slow neutrons (CCN, TTN) generatedby the moderators (CNM, TNM) of different energy spectra are integratedin common in an overlapping neutron beam (SBL) with a multi spectrumdefined by the structure and number of the moderators (CNM, TNM).
 2. Theneutron-optical component array according to claim 1, characterized bythe fact that in case of a direct overlapping of the radiationdirections they are combinable by a predetermined encoding scheme at theplace of experiment.
 3. The neutron-optical component array according toclaim 1, characterized by the fact that the neutron guide (NGT) iscoated with nickel on its internal surface (INS).
 4. The neutron-opticalcomponent array according to claim 1, characterized by the fact that incase of overlapping of the radiation directions by furtherneutron-optical components for obtaining an effective mean radiationdirection of the overlapping neutron beam a further neutron-opticalcomponent is structured as an oscillating reflector which oscillated insynchronism with the pulsed neutron source or with the chopped neutronbeam of a continuous neutron source.
 5. The neutron-optical componentarray according to claim 1, characterized by the fact that in case ofoverlapping of the radiation directions (CBL, TBL) by furtherneutron-optical components (NOC) for obtaining an effective meanradiation direction (EBL) of the overlapping neutron beam (SBL) afurther neutron-optical component (SSM) is provided with an energydependent switching function.
 6. The neutron-optical component arrayaccording to claim 5, characterized by the fact that the furtherneutron-optical component (NOC) is structured as a neutron reflector(SSM) with an energy dependent switching function which by acorresponding angular alignment continuously or intermittently passes orreflects impinging neutrons as a function of their energy.
 7. Theneutron-optical component array according to claim 5, characterized bythe fact that the neutron reflectors (RSM, SSM) are structured in a selfsupporting form or in a form coated on a neutron transparent substrateas a single or multi-layered neutron reflector, with the coating beingapplied to one of both sides of the substrate.
 8. The neutron-opticalcomponent array according to claim 4, characterized by the fact that thefurther neutron-optical components (NOC, RSM, SSM) are integrated intothe neutron guide (NGT).